1. Field of the Invention
The present invention relates to a long-range surface plasmon resonance device utilizing a nano-scale porous dielectric and a method of fabricating the same, and more particularly, to a long-range surface plasmon resonance device having high resolution and high sensitivity by properly adjusting a refractive index of a porous dielectric and a size and density of porosities formed in a porous dielectric layer and to a method of fabricating such a long-range surface plasmon resonance device.
2. Description of the Related Art
Fluorescence analysis has been widely used as a biosample analysis method. According to the fluorescence analysis, each of the biomolecules is first colored by a fluorescent dye having a typical reaction wavelength and information such as an ingredient of the biosample is then analyzed from a spectrum of light emitted from the sample by irradiating light to the biosample mixed with a variety of biomolecules. However, the fluorescence analysis has problems in that the coloring process for the biosample is complicated and the fluorescent dye is very expensive. To solve these problems, a variety of methods for analyzing the biomolecules without using the fluorescent dye have been developed. One of them is a method using surface plasmon resonance. The plasmon is a kind of surface electromagnetic waves traveling along interface surfaces between a thin metal layer and a dielectric and a surface plasmon resonance phenomenon is produced by a charge density oscillation generated on a surface of the thin metal layer.
FIG. 1 shows a conventional structure incurring such surface plasmon resonance.
Referring to FIG. 1, a prism 10 and a thin metal layer 12 are respectively attached on bottom and top surfaces of a transparent substrate 11 and a fluid sample 13 to be measured is disposed on the thin metal layer 12. Here, the transparent substrate 11 and the prism 10 are formed of materials having the same refractive index. As shown in FIG. 1, when light is directed to the boundary surface between the thin metal layer 12 and the transparent substrate 11 at an angle greater than a total reflection angle, a total reflection is generated. Thus, an evanescent wave having a very short effective length is generated and advances from the reflective surface to the thin metal layer 12. Since a thickness of the thin metal layer 12 is less than the effective length of the evanescent wave, the evanescent wave can reach the liquid sample disposed on the thin metal layer 12. At this point, when the wavelength of the incident light is continuously varied, the light is absorbed at a specific wavelength and a charge density oscillation appears on the surface of the thin metal layer 12. This is called an excitation of the surface plasmon. This phenomenon may be generated at a specific incident angle when the light incident angle is continuously varied instead of varying the wavelength. The wavelength or incident angle when the surface plasmon is excited is determined by the refraction index of the liquid sample 13.
FIG. 3 shows a graph illustrating variation of the reflectivity according to the variation of the wavelength. A reflectivity curve indicated by the reference character A is a case where the liquid sample 13 is water. This shows that the surface plasmon resonance is generated at a wavelength of about 700 nm and the reflectivity is steeply reduced. When the refractive index varies by dissolving a biomaterial in the water, the resonance wavelength varies as indicated by a reflectivity A′. When this principle is used, it becomes possible to detect a specific biomolecule from the liquid sample.
However, when this method is used, since the curve variation of the reflectivity is very small and a width of the curve is wide, the resolution and sensitivity are not enough high. In addition, since the effective distance of the evanescent wave is very short, it is difficult to measure a relative large sample.
FIG. 2 shows a structure for solving the above-described problem. In this structure, a buffer layer 14 formed of dielectric is disposed between a transparent substrate 11 and a thin metal layer 12. In this case, since the effective length of the evanescent wave is increased, it becomes possible to measure a relative large sample. This is called a long-range surface plasmon resonance. In addition, as can be noted from reflective curves B and B′ of FIG. 3, since a very sharp reflective curve is formed, higher resolution and sensitivity can be obtained. In FIG. 3, the reflectivity curve B is for a case where the liquid sample 13 is pure water and the reflectivity curve B′ is for a case where there is refractive index is varied by adding other material to the water. When comparing the reflectivity curves B and B′ with the reflectivity curves A and A′, it can be noted that the curves B and B′ are very sharp.
However, there are very few dielectric materials that can be used for the buffer layer 14. That is, the buffer layer 14 should be made of a transparent material that can be coated on the transparent substrate 11 while having a refractive index similar to that of the liquid sample so that the surface plasmon resonance can be generated. There are only two materials, Teflon and MgF2 that can satisfy the above conditions. However, since these materials have fixed refractive indexes, an optimal reflectivity curve cannot be provided according to the liquid sample. That is, even when the reflective curve becomes sharp, noise increases by outer conditions, thereby making it difficult to actually improve the resolution.